This project is a longitudinal study of a number of parameters of aging utilizing a colony of C57BL/6 mice. Studies being conducted include a) assessment of age-related changes in the micro-environment of hematopoietic stem cells to determine if such changes may contribute to defects in proliferation and differentiation responses of old stem cells b) characterization of the intestine of the aging animal by measurement of cells, crypts, cell migration rates and determination of the generation, growth, and survival curves for crypt stem cells c) determination of the effect of age on liver microsomal aminopyreine demethylase activity and aniline hydroxylase activity d) determination of the activity of adenosine and purine nucleoside phosphorylase with age in liver and lymphocytes e) determination of liver asparagine synthetase activity and determination of the ratio of high molecular weight to low molecular weight forms of the enzyme as a function of age f) examination of sexual behavior and reproductive function as indices of the aging process g) determination of the effect of a growth-hormone-like factor produced by Spirometra mansonoides larvae on the age-related decline of immune function and interferon production.